There is a requirement for screening of cargo items for detection of illicit materials and devices. Among these illicit substances are special nuclear materials such as Uranium and Plutonium.
It is known that such materials can be detected using radiation detectors operating in a pulse counting mode where the detector is selected for the measurement of gamma radiation, typically in the energy range 0.05 MeV to 2 MeV.
These devices are typically operated in a portal design 10 in which large slabs of radiation detector material 12 are suspended either side of a measuring area and an item 14 under inspection is driven between the radiation detectors as shown in FIG. 1, in order to attempt to detect any radiation source 16.